Natural circulation start-up system for once-through steam generator



April 7, 1970 w. P. GORZEGNO NATURAL CIRCULATION START-UP SYSTEM FORONCE-THROUGH STEAM GENERATOR Filed Oct. 11. 1967 INVENTOR. WALTER I?GORZEG/VO RICHARD H. THOMAS ATTORNEY United States Patent 3,504,655NATURAL CIRCULATION START-UP SYSTEM FOR ONCE-THROUGH STEAM GENERATORWalter P. Gorzegno, Florham Park, N.J., assignor to Foster WheelerCorporation, Livingston, N.J., a corporation of New York Filed Oct. 11,1967, Ser. No. 674,550 Int. Cl. F22d 7/00 U.S. Cl. 122406 15 ClaimsABSTRACT OF THE DISCLOSURE A once-through vapor generator includingmeans for operating the generator during start-up by natural circulationsubsequently converting to forced circulation during high pressurenormal operation of the generator.

This invention relates to improvements in the starting of a once-throughvapor generator.

To start-up a once-through boiler, it has heretofore been necessary toprovide a by-pass line around the turbine or point of use, and/orportions of the generator to handle the flow during the start-up period.The reason for this is that during start-up, the fluid is in either aliquid state or liquid-vapor mixture state neither of which can behandled by the turbine, and perhaps portions of the generator. Towardsreducing start-up time, it has been the practice to provide a reducedpressure flash tank in the by-pass line designed to provide a vapor flowearlier in the start-up period for warming and rolling the turbine forother uses.

Many disadvantages are experienced with these conventional systems, aprincipal one being that a separate source of power is required to drivethe feed pump during the initial stage of start-up and before vapor isproduced in the generator. Another disadvantage is that the conventionalby-pass and heat recovery system and controls therefor are complex andexpensive, and by virtue of the by-pass, some surfaces of the generatormay not be cooled during the startup period. A further disadvantage isthe difliculty of matching fluid enthalpy in conventional units duringchangeover from flow through the by-pass line to flow to the point ofuse or turbine. Usually a higher quality flow is obtainable from theflash tank than the main flow path at the time of switchover to the mainflow path, the result being a sudden temperature drop at the point ofuse.

In accordance with the invention, these disadvantages are overcome byproviding a means for natural circulation of the boiler during start-upwith change-over to forced circulation at a predetermined load. Toaccomplish this, the tubes of the generator in the furnace sectionthereof are parallel to each other and vertically oriented. Circuitinlet and outlet headers are provided from which riser tubes connect tomanifolds at the top of the generator; a drum is positioned connected tothese manifolds to receive the flow therefrom. Separation means in thedrum separate the flow into vapor and liquid streams, and an unheateddowncomer disposed outside of the furnace section returns the liquidstream to the tube inlet headers. The downcomer is sized large enough sothat the total circuit resistance to flow during start-up of thegenerator is sufiiciently less than the head created by the differencein densities of the flows in the riser tubes as compared to the unheateddowncomer to obtain unassisted natural circulation of the flow. At apredetermined load point, for instance approximately 30% load, naturalcirculation flow in the unheated downcomer changes to a forced flow;that is suitable feed pump means forces the flow through the circuits ofthe generator. A by-pass line is provided by-passing the drum.

ice

The invention and advantages thereof will become apparent uponconsideration of the following specification with reference to theaccompanying drawings, in which the figure is a vertical perspecive andpartial section view of a multi-pass vapor-generator in accordance withthe invention.

Referring to the figure, there is illustrated a vaporgenerator 12 havinga furnace section 14 defined by riser tubes 16. Suitable inlet headersconnected to tubes in the front and rear walls (18a and 18brespectively) of the generator make these walls a first flow pass in thegenerator, and suitable inlet headers feeding the side walls (20a and20b) of the generator (plus portions of the front and rear walls) makethese walls the second furnace flow pass. A main feed pump 22 isoperatively connected (in a way to be described) to the first pass walls18a and b, and downcomer 24 recycles the flow from the top of thefurnace front and rear walls to the side walls 20a and b via valve 26;for two-pass furnace operation of the furnace.

The normal once-through flow in the boiler is then from the feed ump 22to the economizer 28; to a generator drum 30 (to be described) via line31, then via downcomer 32 to the front and rear walls (18a and b) of theboiler, and finally through downcomer 24 to the side walls (200 and b)of the boiler, with valve 26 open.

A once-through steam generator during start-up must maintain a minimumcircuit flow in the furance for adequate cooling. Initially the turbinecannot accept this minimum flow (about 25 to 30% of full load flow),either in quantity or fluid state (enthalpy, pressure), and a startupby-pass system must be employed from initial start-up to 25 or 30% offull load.

The described invention establishes a natural circulation loop for thefurnace circuits during initial start-up and to 25 to 30% load, with theremainder of the circuits cooled by steam generated as a result ofburner input to the furnace. During this start-up load period, as with anatural circulation steam generator, operating pressure may bemaintained at low valves (about 1000 p.s.i. for a 3500 p.s.i. full loadoperating pressure), and steam may be generated by control of firingrate to satisfy turbine and other system demands. In thi way, thestart-up bypass system (required of conventional once-through steamgenerators) is not needed, nor are the complicated controls usuallyassociated with one-through by-pass systems. Feed pump 22 output is onlyrequired to maintain throughput steam flow to meet demand requirements.At the same time steam cooling of surfaces other than the furnace risertubes is easily achieved using conventional natural circulation steamgenerator principles.

Operation during start-up is described as follows: A water level isestablished in drum 30, and shunt line 34 between downcomers 32 and 24,with valve 36, is opened so that the flow in downcomer 32 is into all ofthe passes 18 and 20 (a and b), feeding in parallel the side, front andrear walls. For this purpose valve 26 in line 24 (connecting the passesin series) is closed. On firing the burners in the furnace section ofthe generator, an upward natural circulation for the fluid in thefurnace walls is established, the flow passing from the walls to drum 30positioned at the top of the generator. This is accomplished throughcollecting headers 38 and 40 at the top of the furnace and valves 42 and44 between these headers and the drum 30. At this stage of the generatoroperation, the valves 42 and 44 are of necessity open, as well as valve36 in shunt line 34, mentioned in the foregoing.

In the drum 30, vapor-liquid separators are disposed designed to produceseparate vapor and liquid streams, with the drum having a liquid spaceand a vapor space; Liquid is returned from the drum 30 by means of thedowncomer 32, and the vapor flows via line 46 and valve 58 to remainingheat recovery portions of the generator, including roof 48, convectionwall passes 50 and superheater 52.

A significant feature of the present invention is that the downcomer 32and other components, for instance the drum 30, are sized sufficientlylarge to lower the resistance in the circuits such that up to about 30%load, the difference in densities between the flows in the riser furnacecircuit tubes and the flow in the downcomer is sufficient to achieveunassisted natural circulation of the flow in the boiler.

To achieve forced circulation at the 30% load point, programmedmanipulation of circuitry valves 36, 26, 42, 44, 56 and 58 willgradually change furnace circuit flow from a natural circulationcharacteristic cooling all four walls, 18a, 18b, 20a, 20b, in parallelto a forced circulation characteristic cooling walls 18a and 18b inseries with walls 200: and 20b.

Referring to Table I, the programming for valve operation is evident.

TABLE I.PRO GRAMMED VALVE OPERATION This programmed manipulation ofvalves is as follows:

During natural circulation operation up to 30% load, valves 36, 42, 44and 58 are open, and valves 26 and 56 are closedas listed in Table I.

At 30% load or the load corresponding to the minimum circuit flowrequired by the steam generator for oncethrough operation, theturbine-generator can accommodate 100% of the through-put flow, and theturbine by-pass requirement is zero. At this load point, it is thendesirable to change to one-through operation for the steam generator.

To accomplish this, valve 36 (in shunt line 34), valve 42 between firstpass header 38 and drum 30, valve 44 between second pass header 40 anddrum 30, and valve 58 in transfer steam line from drum 30 to the roofcircuit 48, are programmed closed at a desired rate. Simultaneouslyvalve 26 in downcomer 24 and valve 56 in line 54 are programmed open.For once-through operation feed flow in conduit 31 flows through drum 30to enter downcomer 32 which feeds the first furnace pass, 181: and 18b.

Functional considerations during this transition from naturalcirculation to once-through operation are as follows: At the 30% loadpoint with the furnace circuitry still cooled using natural circulation,the furnace circuit pressure has reached approximately 2500 psi. Thefluid (steam-water mixture) exiting from the furnace circuits at thisstage of operation is about 25% steam by weight at an enthalpy ofapproximately 825 B.t.u. per pound. The circulation ratio for thisnatural circulation loop is approximately four to one; that is, theweight of steamwater mixture circulating in the furnace cincuit loop isfour times the through-put steam leaving drum 30 through line 46.

When the circuit valves listed in Table I are programmed opened andclosed respectively to change from natural circulation to once-throughcircuit flow, the circulation ratio through the furnace circuitsdecreases from four to one, and the exit fluid enthalpy increases toapproximately 1130 B.t.u. per pound. During this changeover period, thefurnace circuits, and other circuits of the steam generator, are alsopressurized to the full pressure level set by design 3500 psi. for asupercritical unit. The once-through fluid mass flow rate in the furnacecircuits is now approximately 600,000 lb./hr. sq. ft. at the 30% loadpoint. Operation to full load is now in accordance with conventionalprocedures for a once-through steam generator.

The 30% load point is usually recognized as the point in once-throughcirculation where a minimum mass flow for proper furnace cooling isestablished. As is well known, the unheated external downcomers in anatural circulation boiler carry saturated water from the drum todistribution pipes and headers feeding the furnace wall risers, whichreceive heat. Natural circulation occurs since the risers contain asteam and water mixture which is less dense than the saturated water inthe downcomers.

As the pressure in the boiler is increased, the difference in densitiesof the mixtures in the risers and downcomers becomes smaller, with lessavailable force or head to promote natural circulation. At lowerpressures, for instance 2,000 pounds per square inch throttle pressurethere is more than adequate natural pressure available and circuitdesign is not highly critical. At higher pressures, such factors assteam in the water in the downcomers, and improper sizing of thedowncomers and/or valves, can more easily retard natural circulation.Accordingly, sizing of the downcomers and valves, or calculation ofresistance in the unit is critical towards maintaining sufficient headup to the pressure at 30% load and switchover.

It is, of course, possible to design the boiler components for naturalcirculation up to loads exceeding 30% but for high subcritical pressurescosts become prohibitive. For instance, drum 30 would have to beinordinately large for minimal resistance to flow. A changeover at orabout 30% load appears most economical in most instances. Changeoverbelow 30% load is feasible, but would require the by-pass system which apurpose of the present invention is to avoid.

In the foregoing, it was mentioned that a very substantial and expectedreduction in fluid mass flow rate is experienced during changeover fromnatural to forced circulation. Accordingly, it may be desired to disposein at least some of the furnace circuits orifices or turbulators in thetubes to obtain equal distribution of the flow and adequate cooling. Inthe present example, by dividing the furnace section into two passes inseries relationship for once-through operation, the control ofdistribution of flow becomes less critical, but it is still feasible, ifthe boiler is of substantial perimeter, to use distribution devices. Asthese devices increase the resistance in the circuit, the extent towhich natural circulation occurs is also decreased. However using alarger downcomer can compensate for increased riser rsistance to obtaina desired circulation ratio.

Although the invention has been described with respect to specificembodiments, many variations within the scope of the following claimswill be apparent to those skilled in the art.

What is claimed is:

1. A method for starting-up and operation of a vapor generator of thetype which includes a flow circuitry including primary radiant heatedsurfaces, secondary primarily convection heated surfaces in series flowrelation-- ship with the primary surfaces, the primary surfaces at leastin part comprising parallel vertically oriented tubes defining thegenerator furnace section; comprising the steps of establishing anatural circulation flow through said primary surfaces;

separating the flow into a vapor stream and a liquid stream;

recycling said liquid stream to the primary surfaces for obtaining saidnatural circulation therein;

passing said vapor stream to said secondary surfaces;

and

at a predetermined load point during start-up causing all of the flowfrom said primary surfaces to pass to said secondary surfaces,circulation following said predetermined point being pump assisted.

2. A method for starting-up and operation of a vapor generator of thetype which includes a flow circuitry including a primary radiant heatedsurface, and secondary primarily convection heated surface in seriesflow relatiOnShip with the primary surface, the primary surface at leastin part comprising parallel vertically oriented tubes defining thegenerator furnace section, the tubes having lower inlet ends and upperoutlet ends; comprising the steps of establishing a fluid level in saidprimary surface;

imparting heat to said fluid to establish a natural circulation risingflow in the tubes of the primary surface;

separating the flow from the tube outlet ends into a.

vapor stream and a liquid stream;

downwardly flowing said liquid stream to said tube inlet ends tomaintain in part said liquid level; passing the vapor stream to saidsecondary surface;

and

at a predetermined load point during start-up causing all of the flowfrom said primary surface to pass directly to said secondary surface,circulation in said generator following said predetermined point beingforced flow pump assisted.

3. A method for starting-up and operation of a vapor generator of thetype which includes a flow circuitry including vapor generating andsuperheating surfaces, the generating surfaces comprising at least inpart parallel vertically oriented tubes defining the generator furnacesection, the tubes having lower inlet ends and upper outlet ends; saidcircuitry further including a fluid collecting and separation drumdisposed above said vertically oriented tubes and connected to the tubeoutlet ends to receive the flow therefrom; comprising the steps ofestablishing a liquid level in said drum and vertically oriented tubes;

heating said tubes to cause a rising flow of the liquid therein;

collecting the flow in said drum from the outlet ends of the tubes andseparating the flow into separate vapor and liquid streams;

downwardly flowing said liquid stream to the tube inlet ends, theresistance to flow upwardly in the tubes and downwardly to the tubeinlet ends being maintained sufliciently low so that the flow is bynatural circulation;

passing the vapor stream from said collecting and separation drum toremaining surfaces of the generator; and

at a predetermined point during start-up causing all of the flow fromsaid primary surface to pass directly to remaining surfaces of saidgenerator, circulation in said generator following said predeterminedpoint being pump assisted.

4. A method according to claim 3 wherein said predetermined point oftransition from natural circulation to pump assisted circulation occurswhen the resistance to adequate flow starts to exceed the head providedby the difference in density of said rising furnace section tube flowand said downwardly flowing liquid stream.

5. A method according to claim 4 wherein the mass flow and pressure inthe generator during natural circulation of the flow are maintained at aportion of normal operation mass flow and pressure, the mass flow andpressure increasing with increase of enthalpy of the fluid being heated,the increase in mass flow correspondingly increasing said resistance toflow, the increase in pressure reducing the head available.

6. A method according to claim 3 wherein said point of transition is ator above approximately 25% load.

7. A method according to claim 3 wherein said furnace section tubes aredivided into at least two discrete upflow flow passes, said methodfurther comprising the steps of causing the flow following transition topump assisted circulation to pass through said upflow flow passes inseries.

8. A once-through generator circuit comprising a plurality of verticallyoriented parallel riser wall tubes having lower inlet ends and upperoutlet ends defining a furnace section;

burner means in the furnace section;

drum means positioned above the furnace section connected to the risertube outlet ends to receive the heated fluid from said tubes;

separation means in said drum means to separate the heated fluid intoseparate vapor and liquid streams, said drum having vapor and liquidspaces to receive said vapor and liquid streams;

a feed system connected to the riser tube inlet ends including pumpmeans to force a cooled flow through the riser tubes;

unheated downcomer means outside said furnace section connected betweenthe drum liquid space and the tube inlet ends;

said downcomer means being sized with respect to the resistance to flowin said circuit whereby the resistance to flow therein is sufficientlyless during start-up of the generator than the head created by thedifference in density of the flows in the riser tubes and downcomermeans to obtain an unassisted natural circulation of the flows therein;

valve means associated with at least said drum means to prevent the flowof heated fluid therein during pump assisted operation of the generator.

9. A once-through vapor generator circuit according to claim 8 whereinthe wall riser tubes are welded together to form gas-tight panels;

header means for said panels connected to predeter mined numbers oftubes dividing the panels into at least two distinct upflow furnace flowpasses;

conduit means connecting said header means so that the passes are inseries flow relationship;

valve means in the conduit means operable during natural circulationstart-up of the generator to prevent the flow therein so that the flowpasses are in parallel flow relationship.

10. A once-through generator circuit comprising a plurality ofvertically oriented parallel riser wall tubes having lower inlet endsand upper outlet ends defining a furnace section;

burner means in the furnace section;

the riser tubes being welded together to form gas-tight panels;

inlet and outlet header means for said panels connected to predeterminednumbers of tubes dividing the panels intoat least first and lastdistinct furnace upflow flow passes;

drum means positioned above the furnace section connected to the risertubes outlet header means to receive the flow from said riser tubes;

separation means in said drum means to separate the flow into separatevapor and liquid streams, said drum having vapor and liquid spaces toreceive said vapor and liquid streams;

a feed system to supply flow to said riser tubes;

pump means connected to said feed system for forced circulation of flowthrough said riser tubes;

unheated downcomer means outside said furnace section connected betweenthe drum liquid space and the tube inlet header means such thatunassisted natural circulation can be obtained;

conduit means connecting said header means so that the upflow passes arein series flow relationship during normal operation of the generator;

valve means operatively associated with said conduit means and feedsystem so that the flow in said upflow passes is in parallelrelationship during start-up of the generator.

11. A circuit according to claim 10 wherein said furnace passes are twoin number.

12. A circuit according to claim 10 wherein said pump means disposed inflow communication with said drum means liquid space and including valvemeans to operatively isolate said drum means from the circuit except forthe flow of feedwater during pump assisted operation of the generator.

13. A vapor generator comprising a primary circuit which includesvertically oriented parallel furnace tubes, collecting and separationmeans adapted to receive the OW from said furnace tubes and to dividethe flow into separate liquid and vapor streams, and downcomer means toreturn the liquid stream to said furnace tubes;

means to impart heat to said furnace tubes;

said circuit being sized so that the total circuit resistance duringstart-up is less than the head created by the diiference in density ofthe flow in said furnace tubes and said liquid stream to obtain anunassisted natural circulation of the flow in said circuit duringstart-up;

pump circulating means for said generator; and

conduit and valve means associated with said circuit and pump means foronce-through circulation of the flow during normal operation of thegenerator.

14. A vapor generator circuit comprising a plurality of verticallyoriented parallel riser t-ubes having lower inlet ends and upper outletends defining a furnace section; means to impart heat to said risertubes; collecting and separation means above the furnace sectionconnected to the riser tube outlet ends to receive the heated fluid fromsaid tubes, and to separate the heated fluid into separate vapor andliquid streams;

means to recycle the liquid stream to said tube inlet ends;

said circuit being sized so that the total circuit resistance duringstart-up is less than the head created by the difference in density ofthe flow in the riser tubes and the liquid stream being recycled to thetube inlet ends, to obtain an unassisted natural circulation of theflow;

pump circulating means for said generator;

valve means for said circuit whereby the flow during pump assistedcirculation is once through the circuit.

15. A vapor generator comprising a plurality of vertically orientedparallel riser wall tubes having lower inlet ends and upper outlet endsdefining a furnace section;

burner means in the furnace section;

the riser tubes being welded together to form gas tight panels;

inlet and outlet header means for said panels connected to predeterminednumbers of tubes dividing the panels into at least first and seconddistinct furnace upfiow flow passes;

drum means positioned above the furnace section;

separation means in the drum means producing separate liquid and vaporstreams;

downcomer means connected to said drum means to receive said liquidstream;

vapor conduit means connected to said drum means to receive said vaporstream;

a feed system including pump means;

said generator including first circuit connections comprising conduitmeans to transmit the flow from said furnace section outlet header meansto said drum means, and conduit means to transmit the flow from saiddowncomer means to said furnace section inlet header means;

said drum means, furnace section downcomer means and first circuitconnections being sized so that the circuit resistance during start-upis less than the head created by the difference in density between theflow in said furnace circuit and said liquid stream to obtain anunassisted natural circulation of the flow during start-up;

said generator including second circuit connections including seconddowncomer means connecting predetermined header means so that saidupfioW passes are in series flow relationship, bypass conduit meanstransmit the flow directly from One of said outlet header means to saidvapor conduit means bypassing said drum means, and means connecting saidfeed system to one of said inlet header means;

valve means for said downcomer and conduit means so that the flow is inseries in said furnace passes and once-through in said generator duringnormal operation thereof.

References Cited UNITED STATES PATENTS 1,707,920 4/1929 Norton l224062,869,517 1/ 1959 Leeberherr 122-406 3,003,479 10/ 1961 Bock et a1.

3,135,243 6/ 1964 Schroedter.

3,215,126 11/1965 Sprague l22406 3,234,920 2/ 1966 Kemmetmuller et a1.l227 3,299,860 l/ 1967 Svendsen l22406 3,368,533 2/1968 Knizia l22406KENNETH W. SPRAGUE, Primary Examiner

